The invention relates to a damping device for reducing the vibration amplitude of acoustic waves for a burner in an internal combustion engine.
During the combustion of fuel in combustion chambers which are used, for example, in aircraft engines or in burners for operating thermal power stations, preferably in gas-turbine plants, the occurrence of so-called combustion-chamber pulsations, which form as acoustic waves, is known and it is attempted to specifically suppress said pulsations by suitable design measures. For example, in afterburner systems in aircraft engines, so-called back-purged aperture plates are used as walls, and these aperture plates serve both to cool the wall and to dampen the acoustic waves occurring inadvertently.
Such back-purged aperture plates are likewise used in conventional gas-turbine combustion chambers and in principle perform the same task in the latter, namely to cool the combustion-chamber wall and specifically suppress acoustic vibrations forming inside the combustion chamber.
In the course of the optimized design of combustion chambers with regard to the reduction in the pollutant emission, the combustion chambers themselves are increasingly designed without cooling-air feeds into the combustion chamber, since all the air is required for the low-pollution combustion. This design, due to the reflecting walls, results in very low acoustic damping, so that such combustion chambers are often provided with additional damping elements.
As a rule, the damping elements work according to the principle of the so-called Helmholtz resonator. Helmholtz resonators are in principle volume elements, the resonance behaviour of which may be set in such a way that they specifically dampen mechanical or acoustic waves of certain frequencies which pass through them.
Approaches are known with which the suppression of acoustic waves inside combustion chambers has been attempted with the use of Helmholtz resonators. In this case, Helmholtz resonators were arranged in the so-called combustion-chamber dome next to the actual burner, as a result of which, on the one hand, the amplitude of the acoustic wave can be attenuated; however, it is not possible in this way to completely reduce the direct effect of the burner on the generation of acoustic waves.
The object of the invention is therefore to develop a damping device for reducing the vibration amplitude of acoustic waves for a burner for operating an internal combustion engine, preferably for driving a gas-turbo group, which burner normally provides a mixing region, in which an air flow and a fuel flow are mixed with one another to form an air/fuel mixture, and a combustion chamber, which in the direction of flow of the fuel/air mixture is arranged downstream of the mixing region, in which the fuel/air mixture can be ignited, in such a way that any acoustic vibrations occurring inside the burner are to be more or less largely suppressed. The damping device according to the invention is to provide possibilities for subsequent fitting in existing internal combustion engines and is to permit easy tuning of the resonance behaviour to the respective burner.
According to the invention, a damping device for reducing the vibration amplitude of acoustic waves and a burner for operating an internal combustion engine is developed owing to the fact that a Helmholtz resonator is directly connected to the mixing region of the burner in such a way that acoustic waves forming in the burner are suppressed in the Helmholtz resonator and are not reflected back into the burner.
The idea underlying the invention is the direct integration of a Helmholtz resonator in the burner itself, so that the acoustic waves produced inside the burner can be completely absorbed by the Helmholtz resonator, which is directly connected to the combustion chamber itself via the mixing region. In this way, the acoustic waves occurring in the interior of the burner are no longer reflected, since the burner, due to the Helmholtz-resonator volume integrated in the burner, has an acoustic adapted rear wall, on which the acoustic waves can no longer be reflected back. This adaptation may also be achieved by means of a quarter-wave volume, as will be explained in more detail further below.
Acoustic feedbacks can be specifically avoided by means of the Helmholtz resonator provided directly in the burner, as a result of which an undesirable feedback of a forming acoustic wave, for example in the region in which the fuel/air mixture is ignited and which is of crucial importance for the conversion of energy, can be completely avoided. Precisely such feedbacks, in combustion-chamber systems of conventional design, lead to undesirable combustion-chamber pulsations, which lead to a considerable decrease in the overall combustion efficiency.
Thus burners which have a conical mixing region, which directly adjoins the combustion chamber, inside which the fuel/air mixture is ignited, have become established for the firing of gas-turbine plants. Such a burner has been disclosed, for example, by EP 0 321 809 B1 and is used with great success for the firing of gas-turbine plants, this publication forming an integral part of the present description. The damping element in the form of a Helmholtz resonator is preferably arranged directly at the tip of the conical burner. The Helmholtz resonator may either be closed on one side or be designed for the passage of supply air and/or fuel.
In order to prevent adverse effects on the acoustic vibration behaviour of the entire burner, which effects may stem from additional fuel or supply-air feed lines into the burner system, such feed lines are preferably to be arranged between the Helmholtz resonator and the burner or the mixing region.
For example, a fuel feed line which is provided in particular for the starting phase and is normally designated as pilot-gas line is attached between the burner and the Helmholtz resonator. Due to the direct proximity between Helmholtz resonator and pilot-gas feed into the air or fuel flow of the burner itself, the damping behaviour of the Helmholtz resonator also acts directly on the action of the additional pilot-gas feed.
In order to be able to individually tune the resonance behaviour of the Helmholtz resonator to the burner, provision is made for the Helmholtz resonator to be longitudinally displaceable relative to the burner. This may be effected, for example, via a telescopic connecting line to the burner or, in the simplest case, via a screw thread, by means of which the Helmholtz resonator and burner inlet may be spaced apart individually.
In a suitable manner, the Helmholtz resonator itself may provide adjusting elements which vary the volume of the Helmholtz resonator and by means of which the resonance behaviour of the Helmholtz resonator may likewise be adapted individually.
The Helmholtz resonator is preferably provided as close to the burner as possible or even in the burner itself. In order to avoid any irritations of the flow with regard to the combustion supply air in the mixing region of the burner, it is advantageous for the Helmholtz resonator to be attached outside a burner dome surrounding the burner. Likewise, measures may be taken to ensure that the Helmholtz resonator is also attached inside the burner casing in an integrated type of construction without impairing the combustion-supply-air flow in the process.
In principle, the provision of a Helmholtz resonator for damping acoustic vibrations inside a burner is not restricted to burner types which provide a mixing region designed in the manner described; burner types which have no swirl-generating central body inside the burner may also be equipped with the damping element according to the invention.
In this context, according to the object of the invention, a quarter-wave damper, as already mentioned above., can be fitted, this damper being based on a unidimensional stationary wave.